Dota2 is essentially a time-optimization game. If you can increase your strength faster than your opponent you will win.
What's important to realize is that pro-players are really good at optimizing. Furthermore they're a team, they play together a lot and they communicate a lot (headsets, not chat). This time efficiency is essential to flash farming.
A lot of things have to come together for flash farming to work:
- The support needs to prepare the stacks. This means s/he can't be in a fight somewhere else on the map.
- Timing, you can only stack on the whole minute mark. From a time efficiency pov this is probably the hardest.
- Some supports can stack two camps at once. I think it's also possible if you have a blink dagger.
MMR is an indication of how strong of a player you are. Dota2 has different elements that you can be strong at, such as: drafting, last hitting, fighting, warding and leadership. Maybe you are really good at last hitting and farming but you're not so good at fighting. If you play a game where your team is fighting a lot, or the opponents come to deny your farm (they roam/smoke your jungle) then you'll have a difficult game.
Heroes like Anti-Mage and Spectre are notorious for requiring a lot of farm. This is because they have very powerful abilities. Anti-Mage is very mobile and can farm fast with blink and battlefury. Spectre can use his ultimate ability to join fights, which is really efficient for farming since he can farm right up to the point he is needed. Denying farm to these heroes is a valid strategy.
I'm around 3.8k and I watched a game of a friend of mine at 1.8k. What I noticed is that even at that low MMR (sub 25 percentile) the players are really good at the game. They understand drafting, items, roles and team strategy. But it's all about efficiency. If you can raise your GPM by 5% , if you can manage to die 1 less time or get 1 more kill in the early game, you'll win more games and your MMR rises.
What's also important to realize is that if your supports don't understand pulling and stacking, the opponents supports also don't understand it, so it's a level playing field. And maybe you do encounter a support that stacks camps, that probably means he's really bad at everything else :P
I can also tell you that even at 3.8k MMR pulling and stacking doesn't matter that much. Having vision and creating space is more important. I have a 5k friend and at that level stacking is expected. Also, jungle farming and pulling was nerfed over the last few patches so it's become less efficient.